Is the "end-of-study guess" a valid measure of sham blinding during transcranial direct current stimulation?

Studies using transcranial direct current stimulation (tDCS) typically incorporate a fade-in, short-stimulation, fade-out sham (placebo) protocol, which is assumed to be indistinct from a 10-30min active protocol on the scalp. However, many studies report that participants can dissociate active stimulation from sham, even during low-intensity 1mA currents. We recently identified differences in the perception of an active (10min of 1mA) and a sham (20s of 1mA) protocol that lasted for 5 mins after the cessation of sham. In the present study we assessed whether delivery of a higher-intensity 2mA current would exacerbate these differences. Two protocols were delivered to 32 adults in a double-blinded, within-subjects design (active: 10min of 2mA, and sham: 20s of 2mA), with the anode over the left primary motor cortex and the cathode on the right forehead. Participants were asked "Is the stimulation on?" and "How sure are you?" at 30s intervals during and after stimulation. The differences between active and sham were more consistent and sustained during 2mA than during 1mA. We then quantified how well participants were able to track the presence and absence of stimulation (i.e. their sensitivity) during the experiment using cross-correlations. Current strength was a good classifier of sensitivity during active tDCS, but exhibited only moderate specificity during sham. The accuracy of the end-of-study guess was no better than chance at predicting sensitivity. Our results indicate that the traditional end-of-study guess poorly reflects the sensitivity of participants to stimulation, and may not be a valid method of assessing sham blinding.